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Recycling Solid Wastes 131
6.6 PURITY OF MATERIALS
Processors and end users of recovered materials typically require that the materials be homoge-
neous and free of contamination. A small amount of an unwanted material may negatively affect the
quality of a recycled product, and may, in some cases, be a hazard to workers. Some industries com-
ply with strict standards as to composition and will not tolerate even very low levels of contamina-
tion. Other industries routinely process materials to remove foreign material.
There is significantly less foreign material in source-separated wastes compared with raw mixed
wastes processed via an MRF. However, many citizens (and politicians) prefer the convenience of
shipping commingled wastes to a central processing facility for sorting into various fractions.
Buyers may also require that the materials be compacted or established under a specific condi-
tion (e.g., bottles are not to be broken, aluminum beverage cans are to be crushed, and HDPE con-
tainers must be baled).
Materials that are commonly recycled or are potentially recyclable are discussed in the follow-
ing sections.

6.7 PAPER
As mentioned in Chapter 4, paper products comprise by far the majority of the municipal solid
waste stream — about 38% of U.S. MSW, more than double than any other component. Paper waste
has its share of environmental and economic costs, as it occupies substantial volume in collection
trucks and landfills. Production of paper waste implies that more trees need to be cut in order to sat-
isfy continued needs for new paper. The large quantities generated along with the associated costs
for disposal provide economic incentives for paper recycling.

6.7.1 PAPER MANUFACTURE
The Chinese developed the first known papermaking process as early as 100 C.E. A suspension of
bamboo fibers was used as the paper base. In the United States, the first paper mill was constructed in
1690 near Philadelphia. Until the mid-1800s, paper was made exclusively from recycled fiber derived
from cotton, linen rags, and waste paper. With an increased demand for paper and paperboard,
techniques for utilizing wood fiber in papermaking were developed (American Paper Institute, 1990;
Tchobanoglous et al., 1993).
Both coniferous and deciduous wood pulp is used in modern papermaking. Hardwoods and
softwoods have very different fiber morphologies, which will therefore result in different paper
properties. The fibers of softwoods are longer and stronger than those of hardwoods (Roberts,
1996); however, softwood fibers tend to form flocs of entangled fibers during the sheet-forming
process, resulting in problems with appearance. To control this effect, softwood fibers are blended
with those of hardwood to provide adequate strength and appearance.
Paper is derived from fibers originating within the cells of land plants; therefore, paper does
not have a fixed chemical composition. Specifically, terrestrial plant cells are mostly composed of
carbohydrate polymers (polysaccharides) incorporated to some degree with lignin, a complex aro-
matic polymer. The amount of lignin commonly increases with the age of the plant. The carbohy-
drate component of the cell contains primarily the structural polysaccharide cellulose. There are
other, lower-molecular weight nonstructural polysaccharides known as hemicelluloses, which play
an important part in pulp and paper properties. There are also relatively small amount of water-
soluble compounds such as alcohols, resin acids and fatty acids, and trace inorganic materials
(Rhyner et al., 1995; Roberts, 1996).
Essential to papermaking, whether utilizing wood fibers or recycled fibers as the base, is that
the fibers be conformable, i.e., capable of being matted into a uniform sheet. They must also be
capable of forming sufficiently strong bonds at the point of contact. Proper conformability and
bonding begin with the pulping process, when the bonds in the wood fibers are broken.